The invention relates to a process for producing a firmly adhering coating on polymethyl metacrylate [sic] (PMMA) substrate surfaces.
In addition to glass, transparent plastics are increasingly being used for the production of optically imaging constructional elements, such as lenses and prisms. PMMA in particular has proven itself in this context. A key disadvantage of the optical components produced from PMMA is their relatively soft surface, which can easily be damaged by scratches. It is therefore common to protect the surface by vapor deposition of quartz (SiO.sub.2) or else by plasma-polymerized silicon compounds. It is also possible to coat the surfaces with optical functional layers in order, for example, to reduce reflection.
It has unfortunately turned out to be the case that the coatings chosen to date do not adhere very firmly. Under thermal stress, the layers become detached or cracks form which, although without disruptive effect in the case of spectacle lenses, for example, are not tolerable in the case of precision optics. In the case of injection molded optical components in particular the so-called spray skin is subject to higher mechanical stress and exhibits poor adhesiveness for applied coatings.
From EP 0 482 615 A1 it is known to provide the surfaces of various polymer materials with a poly(hydroxyethyl acrylate) coating which is intended to provide improved wettability, printability, adhesiveness, antistatic properties and freedom from misting. The special coating is to be produced by one of the known processes of addition polymerization. According to the stated process parameters the surface to be coated is first of all exposed under vacuum to the poly(hydroxyethyl acrylate) in vapor form, after which a plasma discharge is initiated by means of which the coating is polymerized. The coating operation constitutes a final treatment of the polymer surface.